1. Field of the Invention
The present invention relates to a metal-halogen battery, and more particularly it relates to a secondary battery in which zinc is the active material of the negative electrode and bromine is the active material of the positive electrode and in which a particular porous carbon fiber sheet-shaped article is applied onto the positive electrode surface.
2. Description of the Prior Art
Since the energy crisis in 1973, the importance of the energy problem has become widely acknowledged in various fields. While it has become important to develop new energy sources, it has also become important to develop systems which utilize the generated energy effectively, including those for the conversion, storage, transportation, utilization, etc. of energy. Taking the storage as an example, in the large scale generation of electricity such as atomic power generation, coal thermal power generation, etc. which are expected to constitute a large percentage of the electric energy sources in the future, it is necessary for maintaining a high efficiency to generate electricity with a constant output. Therefore a strong requirement for developing a technique of electric energy storage which makes it possible to suitably store the surplus electric energy during the night and release it during daytime in accordance with the variations of energy demand. For example, at present, the yearly working ratio of the main electric power plants is less than 60% in Japan, and there is a continuing decline. Although as a method of storage of electric energy pumping-up hydraulic power generation has been put into practical use, it involves a loss of energy due to transmission, and is becoming limited in the location. Therefore, various other methods, such as new type secondary battery, fly wheel, compressed air, super conduction, etc. are being studied.
Among others, the electrochemical operations using new type battery systems are promising, and these are thought to be the method most realizable for some time to come which replaces the pumping-up hydraulic power generation. And these systems will solve the problems of location and transmission. Also, the new type secondary battery is expected as a backup apparatus for the electric power generation utilizing natural energies such as solar light, wind power, wave power, etc., and it is also expected as the battery for use in electric automobiles. As the secondary battery applicable for the above-mentioned purposes, there have been developed lead storage battery, sodium-sulfur battery, lithium-iron sulfide battery, metal-halogen battery, redox flow type battery, etc. Among these batteries, metal-halogen battery (for example, zinc-chlorine, or zinc-bromine secondary battery) is being developed rapidly in recent years, because of its excellent characteristics such that the battery output is easily regulatable since it is of liquid circulation type; its maintenance control is easy since it is an aqueous solution type battery which operates at low temperatures; the battery capacity can be easily regulated by the electrolyte reserver volume; the active materials of both electrodes are abundant in natural resources and are low-priced; its theoretical energy density is high; and since the battery reaction is simple, the battery is simple in structure and can be made with cheap materials. For example, in the zinc-bromine secondary battery, the active material at the negative electrode; is zinc and the active material at the positive electrode is bromine, and an aqueous ZnBr.sub.2 solution is used as the electrolyte. The electric charge and discharge reactions proceed as follows: ##STR1##
However, in order to put the metal-halogen battery into practical use, there are several problems to be solved. Among others, it is the most important technical subject how rapidly and effectively the reduction reaction of halogen at the positive electrode should be caused, because it influences directly to the energy efficiency of the battery. Examples of inexpensive positive electrodes replacing the conventional Pt plate include a carbon-plastic electrode in sheet-form produced by the heat-press shaping of a mixture of an electroconductive carbon powder and a resin powder, and a sintered carbon plate. With these electrodes, when the discharge proceeds and the concentration of the active material (halogen) at the positive electrode is lowered, it has been usual that the potential drop is remarkable and the energy efficiency of the battery remains at a low value. Especially, upon the electric discharge at a high current density, a marked potential drop has been observed.